1. Field of the Invention
The present invention relates to a method of compressing a multiplicity of form information used for a computer output microfilmer (hereinafter abbreviated to "COM") or the like.
2. Background of the Invention
In the past, a computer output microfilmer has been used for combining variable information, such as revenue and expenditure accounts or the like to be written on trade books, with fixed information, such as lines or the like ruled in forms of trade books. This combined information is used to record composite information on recording media. Such a COM has been described in "Micrographics System" (second edition) by Daniel M. Costigan published by NATIONAL MICROGRAPHICS ASSOCIATION, and also in U.S. Pat. No. 4,293,202.
There have been established two different recording methods. One method comprises the steps of computer processing variable information recorded on a recording medium, such as a magnetic tape, temporarily displaying the processed variable information on a CRT screen, and superimposing the displayed picture on a picture optically read from fixed information recorded on a form slide to thereby record the superimposed picture on a frame of microfilm. The other method comprises the steps of processing variable information by a computer to obtain its picture signal thereof, scanning a form slide by a laser beam to obtain a picture signal of fixed information, and combining the two picture signals to thereby record a composite picture signal on a recording medium.
However, those recording methods have a problem in that a slippage of the record picture, a distortion of the picture or the like, is apt to occur due to external influences or an internal vibration because the form slide is optically read in hard form.
To avoid the defect, there will be suggested a method comprising the steps of previously storing form information as electronic signals in a memory (soft form), and then reading fixed information from the memory at the time of recording by a COM. This method does not directly read information from a form slide.
However, because the record picture used in such a COM generally has a resolution of 2000.times.2500 dots or more per frame, it is difficult to obtain a memory having a storage capacity corresponding to the required resolution. Accordingly, if a memory corresponding to this resolution is used, there arises such a double problem that the apparatus per se becomes large in size and complicated in structure.
In general, form information is composed of frame information such as ruled lines, and fixed information such as customer's name, code number, etc., to be recorded at fixed places. Accordingly, information that should be recorded is sparse and most of the form information is redundant (repetitive) or empty information. Accordingly, if the form information is compressed when it is stored in advance in a memory and the memory is used together with a COM so that the compressed form information is expanded to thereby record such a composite picture of variable information and form information as described above is recorded, it is possible to minimize the capacity of the memory in expectation of the advantages in miniaturization of the COM and lowering of manufacturing cost.
Examples of such information compressing methods, which have been generally used are a run length method, a plane encoding method, a turning-point address encoding method, and the like. In any case, a problem exists in the complexity of the method as well as in low processing speed. Particularly, because COM requires processing in a period of the order of 30 ns per picture element, the usual run length methods present difficulties in that the time allowed is too short to complete compression and expansion. U.S. patent application Ser. No. 853,294, which was filed on Apr. 17, 1986 and assigned to the same assignee as this application, discloses an information compressing method for compressing only one information form, but does not disclose a method for compressing a multiplicity of information forms. It is an important matter how a multiplicity of information forms are compressed, since most forms comprise lines or the like in trade book form which are similar to each other.